1. Field of the Invention
The field of the present invention is watermelon breeding and the genetic improvement of watermelon. More specifically, this application is related to diploid, tetraploid and triploid watermelon seeds and plants for the production of watermelon fruit that (i) have ultra firm flesh and/or liquid-retaining flesh and (ii) are sweet at maturity.
2. Description of Related Art
Watermelon (Citrullus lanatus) is an important commercial member of the Cucurbitaceae family that includes many different varieties. The fruit of these varieties differ in coloring, sweetness, and other traits. For example, watermelon fruit of different varieties display a wide range of coloring on the outside rind. In addition, color in the edible tissue varies from different shades of red to yellow. Watermelon fruit also vary in sweetness, which can be estimated by measuring total soluble solids, or brix, using a refractometer. Because sweetness is especially important to consumers, the U.S. Department of Agriculture has set fruit quality standards based on brix levels (United States Standards for Grades of Watermelon, U.S. Department of Agriculture (1978)). According to these standards, edible parts of the fruit having not less than 8 brix are deemed to be “Good”, while edible parts of the fruit having not less than 10 brix are deemed to be “Very Good.”
Consumers also have the choice of either seeded or seedless watermelon varieties. Unlike the flesh coloring, which is caused by varying genetic loci, the distinction between seeded and seedless varieties is usually caused by human intervention of making crosses that vary ploidy levels. Similar to humans, watermelons are natural diploids with chromosomes arranged in pairs. Many plants, including watermelons, can undergo a duplication of their entire set of chromosomes and exist as tetraploids. While it is uncommon for watermelons to produce spontaneous tetraploids, this process can be routinely produced in the laboratory using cell biology techniques. A tetraploid parent may then be crossed with a diploid parent to produce triploid seeds, which, in turn, generate plants with seedless fruits. In particular, seed formation in the fruit of triploid plants aborts because of the ploidy level differences, resulting in seedless fruits. Many commercial varieties are triploid and seedless.
Fruits of plants of different ploidy also vary in flesh firmness. Diploid lines typically have the lowest fruit flesh firmness levels. For reasons that are unclear, the process of changing a diploid line to a tetraploid line correlates with firmer fruit flesh. In other words, tetraploid lines usually have firmer fruit flesh than diploids. Triploids, being a cross between a tetraploid and a diploid, typically have an intermediate level of flesh firmness.
In addition to consumer preferences as to coloring, sweetness and seeds, there is increasing consumer demand in the fresh produce business for products that combine quality and convenience. Examples of products that meet these criteria are bagged baby carrots, broccoli and cauliflower and bagged leafy crops, such as lettuce and spinach. Similarly, there is demand for mature cut fruits, like watermelon, melon, pineapple, papaya and kiwi. A growing segment of watermelon retail sales are cut fruits that are either displayed in large pieces with the rinds attached, or are cut into smaller pieces, without the rind, and offered to the consumers in plastic food containers. The industry term for these products is “minimally processed.” By 1998, Perkins-Veazie et al. ((1998) Hortscience 33:605) estimated that 10% of the retail watermelon market was minimally processed.
The advantage of such cut fruit displays is that the consumer can visually inspect the quality of the fruit and, in particular, judge whether the fruit is mature and, thus, ready to consume. Often, immature fruits will not be uniform in pigmentation, and overripe fruit will display signs of decay. Moreover, these products offer convenience to the consumer.
The disadvantage to the produce retailer in presenting minimally processed watermelon products is that cut fruits have a short shelf life. Studies indicate that minimally processed products have a shelf life of 2 to 3 days maximum (ibidem; Wehner et al. In: Watermelons: Characteristics, Production and Marketing. Maynard, editor. ASHS Press, Alexandria Va. 2001.)
Watermelon fruits currently available typically undergo rapid quality deterioration after being cut. Cutting the fruit causes decay, which is observed as a softening of the fruit texture. Deterioration is also manifested as liquid leakage; in some varieties, the flesh of a fresh cut watermelon fruit quickly becomes unattractive to the consumer. The rapid deterioration of cut watermelon fruit places both time and space constraints on the retailer. Because cut fruits have a short shelf life, the retailer typically performs the processing on the retail site. In addition, the retailer has to monitor the products often to ensure that deteriorating products are discarded.
Unlike the sweetness standards established by the U.S. Department of Agriculture, there are no industry standards to describe the firmness of the edible portions of watermelon fruits. Therefore, there are a wide range of descriptors in use, from “firm” and “crisp” (Erma Zaden catalog descriptors for varieties Gil 104 and Erma 12) to “very firm flesh” (Zhang et al. in USPTO application numbers 20040060085 and 20030217394 and Seminis watermelon catalog for the variety Cooperstown). Seminis has described cultivars Fenway, Royal Star and Sentinel as having “excellent crispness,” “firm flesh” and “crisp juicy flesh,” respectively. In addition, Rogers Seed Company advertises the Tri-X Brand 626 as “exceptionally firm” and the Tri-X Brand 313 as having “firm texture” and “crispness.”
While advertising terminology used to describe watermelon fruit flesh firmness is quite variable, scientific reports, using quantitative measurements, show that typical commercial germplasm have had substantially lower flesh firmness than the watermelon fruit of this invention. For example, Roberts et al. (2004 Report from: Watermelon Research and Development Working Group. 24th Annual Meeting, Tulsa, Okla.) measured flesh firmness in a wide range of germplasm, using a penetrometer to measure the amount of force resisted. The data were reported in Newtons, an International System of Measurements term. For purposes of comparison with Applicants' penetrometer measurements, Applicants converted Roberts' data to pounds force (lbf), using the following formula: 1 lbf=4.448 Newtons. Roberts reports a range of watermelon flesh firmness between approximately 1.4 to 3.4 lbf. One of the lines analyzed is Rogers Seed Company line Tri-X Brand 313. As noted above, Rogers Seed Company advertises this line as having “firm” flesh. Roberts et al. measured the flesh firmness in Tri-X Brand 313 as 10.84 Newtons, which converts to approximately 2.4 lbf. Applicants also tested the flesh firmness of Tri-X Brand 313, using a penetrometer from QA Supplies in Norfolk, Va. (Model FT011) with a probe diameter of 8 mm. Using this methodology, Tri-X Brand 313 has a flesh firmness reading of 1.4 lbf (Table 1). Because Roberts does not report the size of the penetrometer probe used, Applicants cannot directly compare their data to Roberts'. At least for Tri-X Brand 313, the approximately 77% higher reading measured by Roberts et al. compared with the protocol described herein may be the result of different methodology, and, in particular, the use of differently sized penetrometer probes. Although the Applicants of this invention use an 8 mm probe, another commonly used penetrometer has a diameter of 11 mm, which would account for the different readings, as penetrometer area is approximately 73% higher for an 11 mm probe as compared to an 8 mm probe.
Schultheis and Thompson (2004 Report from: Watermelon Research and Development Working Group. 24th Annual Meeting, Tulsa, Okla.) also survey watermelon fruit flesh firmness. Although these authors use a different model penetrometer than that used by Applicants, they use a very similarly sized probe with a diameter of 5/16″ or about 8 mm. Schultheis and Thompson report that line Tri-X 313 had flesh firmness readings between 1.4 and 1.7, which are similar to Applicants' measurements, shown in Table 1. In this report, however, the authors describe these firmness data in units of pounds/square inch. It is suspected, however, that the units provided in the Schultheis and Thompson report should be in pounds force, as a reading of 1.4 pounds/square inch, using a 5/16″ probe, is only 0.15 pounds force.
Maynard and Sidoti (2003 GCREC Research Report BRA-2003; Univ. Florida, Gulf Coast Research and Education Center, Bradenton, Fla.) report an additional survey of fruit flesh firmness of commercial watermelon lines. In this study, the authors use a different model penetrometer than that Applicants use in the method described herein, with a larger sized probe having a diameter of 7/16″ or about 11 mm. Their firmness data range from 1.8 to 3.0 pounds/square inch. As with the Schultheis and Thompson report, Applicants believe that these authors are using the incorrect units in their firmness readings. Assuming that these data are actually in pound force units, they compare well with the results obtained using the methodology described herein. For example, Maynard and Sidoti's firmness measurement of line Tri-X 313 was 2.6. If one adjusts this figure to correct for the approximate 2 times difference in probe area, the new figure is 1.35, which is nearly identical to Applicants' measurement of this same line, (Table 1). On the other hand, if one assumes that the data are correctly reported in lb/square inch, the figure of 2.6 lb/square inch based on a 7/16″ probe would be reading of 0.39 lbf. The Tri-X 313 line should have a much higher firmness reading than 0.39 lbf, providing further evidence of inconsistency in how such units have been reported in the prior art.
Leskovar et al. ((2004) J. Horticultural Science and Biotechnology 79: 75-81) also report watermelon fruit firmness. Although this manuscript uses a different measurement protocol, the authors describe in detail their methods, allowing the data to be converted for comparison with the data described herein. After converting to the same units, the range of germplasm analyzed had fruit firmness between 0.9 lbf and 1.5 lbf.
Although measurements of the prior art can be confusing, there is clarity that commercial watermelon lines produced prior to this invention have fruit firmness that is well below 3 lbf. In addition, as shown in Example 4, the fruit of such commercial watermelon lines, once cut, undergo significant liquid leakage. The present invention, therefore, addresses the need in the marketplace for watermelon lines that produce fruits that have a longer shelf life when processed. Specifically, the watermelon of this invention have (i) ultra firm flesh, which avoids the problem of cut fruit becoming overly soft, and/or (ii) liquid-retaining flesh, which delays deterioration of cut fruit by liquid leakage. In addition, these fruits have quality characteristics desired by the consumer, such as sweetness and attractiveness, and offer the retailer both flexibility as to where fruit processing occurs and additional shelf life once fruit is processed.